RADIOMETRIC CALIBRATION OF ALS DATA FOR ARCHAEOLOGICAL INTERPRETATION C. Briese, M. Doneus, G. Verhoeven 1. INTRODUCTION Airborne laser scanning (ALS, resp. airborne LiDAR) is a widely used data acquisition method for topographic modelling. Due to its ability to accurately and densely sample the terrain surface, it became a commonly used technique for the genera- tion of digital terrain models (DTM). In archaeology, this po- tential has revolutionised prospection of forested areas (Doneus and Briese, 2006). For the analysis and classification of the to- pography, geometric criteria derived from the acquired 3D point cloud are typically used. Next to the widely used geometric in- formation, ALS systems typically provide additional informa- tion about the recorded signal strength of each echo. In order to utilize this additional radiometric information for the study of the point-wise backscatter characteristic of the sensed surface, radiometric calibration is essential (Briese et al., 2008; Wagner, 2010). As a result of this radiometric calibration (Briese et al., 2008; Lehner and Briese, 2010) physical quan- tities (the backscatter cross section σ [m 2 ], the backscattering coefficient in γ [m 2 m -2 ] and the diffuse reflectance measure ρ d [m 2 m -2 ]) that describe the backscatter characteristic of the sensed object at the specific laser wavelength are available. This publication focuses on the radiometric calibration of ALS data for archaeological interpretation. After the presen- tation of the radiometric calibration workflow, a full-waveform ALS data set from the case study area Carnuntum (Austria) is investigated. Based on the ALS trajectory and the observables estimated from the decomposition of the full-waveform data set (namely the range, amplitude and echo width per detected ALS echo) the complete ALS data set is calibrated radiometrically. Subsequently the results for the case study area are presented and discussed. This section includes the archaeological interpre- tation of the resulting radiometric image that can be estimated from the calibrated diffuse reflectance measure ρ d of the ALS point cloud. The final section provides a conclusion and outlook into future work. 2. RADIOMETRIC CALIBRATION OF ALSDATA The physical basis for the proposed radiometric monochromatic calibration of ALS data is the radar equation (Jelalian, 1992). The practical workflow for absolute radiometric correction based on full-waveform ALS data and in-situ reference targets consists of the following steps (c.f. Figure 1 and Briese et al., 2012): 1. Selection of the in-situ reference targets based on the ALS flight plan 2. Determination of the incidence angle dependent re- flectance ρ d of the reference surfaces utilising a spec- trometer or reflectometer (cf. Briese et al., 2008) that operates at the same ALS wavelength 3. Recording of meteorological data (aerosol type, visibility, water vapour, etc. for the estimation of an atmospheric model) during the flight mission in order to estimate the atmospheric transmission factor 4. Full-waveform decomposition (echo extraction and esti- mation of echo parameters) 5. Direct georeferencing of the ALS echoes and maybe strip adjustment in order to get an advanced relative and abso- lute georeferencing of the ALS data 6. Estimation of the local surface normal in order to consider the local incidence angle 7. Estimation of C cal based on the ALS echoes within the in-situ reference targets (e.g. defined by a polygon area) 8. Radiometric calibration of all echoes based on the deter- mined value of C cal At the end of this workflow, which can be realised with the pro- gram package OPALS (OPALS, 2012), each ALS echo has as- signed the additional calibrated diffuse reflectance measure that can be used in the further radiometric analysis of the ALS data. 3. STUDY AREA In order to study the process of radiometric calibration of ALS data for archaeological interpretation of the scene the case study area Carnuntum (Austria) was selected. The ALS data were collected on the 5 th of June 2010 (ALS sensor: RIEGL LMS Q-680i). This ALS sensor utilises a laser source for the active illumination and signal detection at a wavelength of 1550 nm. Simultaneous to the aerial data acquisition campaign, in-situ radiometric ground control measurements with a reflec- tometer (can be seen as a single band spectrometer; the instru- ment was provided by the company RIEGL) were acquired. The resulting point density (last echo) of the ALS data was higher than 4 points per m 2 . The radiometric processing of the full- waveform data set was performed with the software OPALS. 4. RESULTS AND DISCUSSION Figure 2 illustrates the result of the generation of a radiomet- ric image (diffuse reflectance measure ρ d ) of the complete case study area, while Figure 3 provides a detailed view of the cal- ibrated radiometric information over an archaeologically inter- esting area near Roman Carnuntum’s military amphitheatre. In the lower right part of Figure 3, a first result of an archae- ological interpretation of the radiometric information provided by ALS data is presented. It clearly shows that the delineation of different archaeological features is possible with the help of the estimated radiometric information. 5. CONCLUSION This contribution provides first archaeological results for the usage of calibrated radiometric information derived from full-